As for Sephy killing Aeris, that was a necisarry plot turn that actually saved the game from ending there.
As far as I know, the game could not of ended there. (SPOILER) Cause her prayer worked, Holy is just being suppressed by some momma's boy.

Yeah, a truly good scene. Though I am not a huge fan of Aerith, I was kinda touched by Clouds rection.

Bipper